The Earthworm 273 



RESPIRATION 



The earthworm needs oxygen just as do all animals; but, as it has 

 no lungs, it obtains its oxygen through its moist outer membrane. 

 Immediately beneath the cuticle there are many capillaries which pre- 

 sent a great expanse of blood area somewhat similar to the many capil- 

 laries in the lungs of higher forms. The oxygen here combines with 

 haemoglobin. The blood gets to these capillaries through the vessels 

 supplying the body wall and is then returned to the dorsal trunk by way 

 of the sub-neural trunk and the intestinal connectives. 



As the nervous system must coordinate every movement of the 

 body, it requires an excellent blood-supply, which is furnished the better 

 in the earthworm by the sub-neural trunk lying very close to the ventral 

 nerve cord. The nervous system is thus continually supplied with fresh 

 nourishment. 



THE EXCRETORY SYSTEM 



Most of the excretory matter is carried outside the body by a num- 

 ber of coiled tubes called nephridia, a pair of which lie in each somite 

 except the first three and the last. The dorsal pores also serve as ex- 

 cretory organs to a minor extent. 



A clear understanding of the nephridia is important, because such 

 an understanding will serve in good stead in the study of the excretory 

 organs of vertebrates. This is the better understood when it is known 

 that the excretory organs of all higher forms develop from embryological 

 beginnings quite similar to those of the earthworm. 



Each nephridium (Fig. 168) consists of: 



(1) The funnel or nephrostome ( ), 



(2) The ciliated neck, 



(3) The coiled narrow tube, 



(4) The wide glandular tube, 



(5) The ejaculatory duct opening to the outside. 



The ciliated neck of each nephrostome passes through the anterior 

 wall of the somite, close to the mid-ventral line. Each nephrostome, 

 therefore, lies in the somite directly anterior to the one containing its 

 own nephridium, so that waste matters from any one somite are expelled 

 to the outside by the nephridium of the next posterior somite. The 

 nephrostomes, or mouths, of the nephridia are flattened fan-like struc- 

 tures, consisting of two flattened lamellae or plates, with a narrow slit- 

 like opening between them. The large cells, which line the opening, are 

 covered with powerful cilia which maintain a constant current toward 

 the tubular part of the nephridium. These tubes are developed in coils 

 which lie in the posterior parts of the somites. There are three coils 

 or turns in each. The third ends in an enlarged portion opening to the 



